A key modulator of somatic fetal growth is insulin-like growth factor (IGF) which is synthesized as a paracrine growth factor in many developing fetal tissues. Studies with human fetal tissues show a specific spatial and temporal pattern of mRNA expression of IGF-I and -II and IGF binding proteins (IGFBPs) in various fetal organs, but are limited to brief periods during gestation (i.e., 8-14 weeks). To fully assess the role of these essential peptides in primate growth, we conducted a longitudinal study on the expression of IGF-II and IGFBP-3 genes in a nonhuman primate model, particularly during the periods of growth acceleration. Tissues (liver, kidney, placenta) were collected from rhesus monkey fetuses roughly every two weeks from gestational day (GD) 65 (early second trimester) through GD 155 (term) (N=50), then processed for in situ hybridization using radiolabelled human probes. IGF-II mRNA was abundantly expressed in fetal kidney (maturing glomerulus, supporting mesenchyme), liver, and placenta (vilus syncytiotrophoblast, basal plate), with a decline noted close to term (GD 150). IGFBP-3 mRNA was more modestly expressed within the kidney (nephrons, collecting system mesenchyme) and liver; a marked decrease in the hepatic expression of IGFBP-3 mRNA towards the end of the second trimester was noted. IGFBP-3 expression in placenta (mesodermal core, anchoring villi, basal plate) did not change appreciably during the time points evaluated. These studies have shown that (1) IGF-II and IGFBP-3 are expressed in specific cell types of the fetal monkey indicating a paracrine/autocrine role during development, (2) changes in IGF and IGFBP mRNA expression occur with advancing gestation, and (3) fetal monkey tissues express IGF-II and IGFBP-3 mRNA in a similar manner when compared to the human, although some species differences in the placenta were noted. *KEY*Fetus, Placenta, mRNA expression, in situ hybridization